FinalHitChance is what the UI displays, and right now all it does is take the pre-graze-band hit chance and subtract dodge demote, which is wrong. Hopefully the actual shot roll doesn't use FinalHitChance, because then the actual hit calculation would be off (rather than just the UI display).

You are correct that crit in general reduces the graze band chances, so using hit + crit + 1/2 (graze) the displayed value is no longer accurate. However, increasing the displayed hit chance (while mathematically correct in some sense) is also misleading in some sense because the chance to miss has not been altered.

Graze band alters the chance to miss though, so I think the current UI is already misleading because showing crit + hit + half graze isn't really intuitive. We're not arbitrarily weighing crit vs hit, so why only half graze? It makes the most sense to me if the UI just shows total crit+hit+graze chance, which is conveniently what existing hit chance display mods assume FinalHitChance is.

The shot wing will need an extra line like "graze band +10%" so the modifiers add up properly. Though short of explicitly showing graze chance (or crit + hit + graze and crit + hit side-by-side) I can't think of a good way to convey to the player that it's also making full damage less likely, and isn't some sort of difficulty handicap.